Have you checked to see if the PS4NET plugin is selected in the netplay menu or if it's in the plugins folder? If you want an easy to use CD plugin, try either Xeven's or SaPu's plugins. If you want something that's more advanced, try using P.E.Op.S., which is an open source continuation of Pete's plugin.
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Main PC:
Asus P6T Deluxe, Intel Core i7 920 C0 @ 3.7 GHz (195x19, 1.3v core, 1.84v PLL, 1.3625v uncore) w/Thermalright Ultra 120 Extreme, 3x2 GB G Skill PC3-12800 @ 780 MHz, 7-7-7-21-1T, 1.66v; ATi Radeon HD 4870 1 GB @ 790/1000 w/Zalman VF-1000, 300 GB WD VelociRaptor, 4x500 GB WD RE2 (RAID 10), 3ware 9650SE-4LPML RAID controller, SB X-Fi Titanium, Seasonic M12D 750w, CM Stacker 810, Windows 7 Pro 64-bit, Dell 2007WFP
XBL/PSN/WoW: Thordrune